This invention generally relates to the construction and operation of acoustic transducer arrays and, more particularly, to the elimination of the odd grating lobes propagated from such arrays.
When a plurality of transducer elements are arranged in a grating-type array and are energized to propagate a steerable acoustic beam, the beam includes a main lobe, a plurality of small side lobes and one or more grating lobes. The grating lobes originate from the acoustic waves that combine along various axes that differ from the axis of the main lobe. Grating lobes are present in the acoustic beams propagated from nearly all linear arrays and normally contain substantial energy.
Grating lobes present a special problem in the technology of acoustic wave imaging because each grating lobe returns a signal that is difficult to distinguish from the main lobe. Normally the side lobes of the beam are of small size and can be eliminated by known signal processing techniques. The grating lobes, however, can have dimensions comparable to the main lobe. Thus, both the grating lobe and the main lobe can give a comparable reflection from the same object of interest.
In the fields of radar and sonar technology grating lobes have been eliminated through various signal processing techniques. In one application the array was pulsed with signals of constant amplitude and the return echo received by the array was mathematically weighted to amplify the amplitude of the center of the return signal. In another application grating lobes were eliminated by randomly spacing the transducer elements along the array. Random spacing eliminated the interference which causes grating lobes but in turn the amount of energy either transmitted or received by the array was reduced. Also, grating lobes have been reduced by driving an array with signals that have a large amplitude at the center of the array and smaller amplitudes at the ends of the array. Although this technique can reduce grating lobes, the main lobe propagated from such an array tends to have an expanded width and becomes fatter. In short there has herefore been no satisfactory solution to the problem of grating lobes that did not cause other effects that seriously degraded the operation of the array.